The present invention in general relates to floating roof storage tanks, and, more particularly, to a seal for use with such tanks.
Evaporation loss is the natural process whereby a liquid is converted into a vapor which subsequently is lost from storage tanks, and evaporation loss occurs only when the vapor reaches the atmosphere. Evaporation loss is a major problem faced by the petroleum industry, and many systems and concepts have been proposed to remedy this problem. Among these proposals is the floating roof which is intended to eliminate the vapor space above the surface of the stored liquid. To further remedy the problems related to evaporation, various forms of seals have been proposed for use with floating roofs. The seals are designed to close the vapor space between the perimeter of the floating roof and the tank shell.
While seals do remedy the evaporation loss problem, modern regulations are increasingly strict. Some of the requirements relating to evaporation loss are directed at the size of the gap which exists between the floating roof seal and the tank wall, and while these gap requirements have been relaxed from a strict "no-gap" requirement, they still remain stringent. Furthermore, it has been theorized that if gaps are present, wind effects can have a more significant influence on evaporation loss than if no gaps are present. Accordingly, improved seals for floating roofs are to be desired.
Among known seals are the metallic seals and the non-metallic sealing rings. However, with the stringent demands placed on the roof seal, these seals have been found lacking. The inventor is aware of several non-metallic seals. For example, U.S. Pat. No. 3,134,501, issued to R. W. Bodley, discloses a seal for a floating roof which includes a pneumatic tube surrounding the floating roof. The pneumatic tube is formed of a non-pliable material having a memory, that is, after deformation, the material having a memory resumes the pre-deformation shape, and therefore tends to resist deformation. The pneumatic tube disclosed by Bodley encircles the floating roof in a collar-like fashion. Such a seal suffers many drawbacks. For example, due to the natural resiliency of the pneumatic tube, and the material memory thereof, the seal cannot conform accurately to the tank wall shape, especially if that shape is not perfectly uniform, and gaps may exist between the seal and the wall thereby vitiating the effect of the seal. Even though the pneumatic seal of the Bodley Patent is more flexible than a metallic seal, the Bodley seal is not completely flexible, and accordingly, cannot effectively deform to fill gaps created by tank wall non-uniformities. Because the seal is supported in collar-like fashion, water from rain, snow, or the like, may collect on top of the seal, thereby creating problems.
There is yet another cause of difficulty in regard to the collar-like support of the Bodley seal. Mill scale, rust and the like from the tank shell and from the roof may fall and collect on top of the seal causing it to be loaded downwardly. In addition, in certain applications, the lower surface of the pneumatic seal will be immersed, and this immersed portion of the seal may change in dimension due to the chemical effect of the stored product on the seal. This chemical effect may cause, for example, a drastic shrinkage of the seal, thereby radically changing the geometry of the seal.
Furthermore, the seal in the Bodley Patent may allow loss from the tank due to permeation through the seal. Because the material of the Bodley seal has a memory and the seal therefore has some stiffness, the seal may ride up and lose contact with the stored fluid, thereby creating a vapor space with the attendant potential evaporation losses. Therefore, while the seal disclosed by Bodley does provide some advantages over other known seals, it still suffers many drawbacks.
Furthermore, in seals such as the Bodley seal, a longitudinal seam is required which must not be permeable, either by the gaseous fluid contained in the seal, or by the vapors from the stored product. Such longitudinal seams often leak and may vitiate the effectiveness of the seal.
It is also noted that, with reference to FIGS. 2 and 5 of the Bodley patent, the upper surface of the pneumatic tube runs essentially horizontally from the attachment point. The lower surface runs vertically downward toward the liquid surface from the attachment point. This means that the only mechanism available for holding the seal in the outward position is the bending stiffness of the fabric used in the seal, or whatever air pressure is present in the seal. While Bodley calls the seal a pneumatic seal, it is not disclosed that the seal is pressurized. Bodley does not disclose any kind of a pressurizing mechanism and there is no suggestion in the Bodley patent that the support of the seal is airtight. Therefore, any air inside the pneumatic tube is of no consequence in supporting that tube. Furthermore, from the Bodley disclosure, the bending stiffness of the material utilized in forming the tube appears to be small. Therefore the pneumatic tube disclosed by Bodley is limited in reach, that is, the dimension between the floating roof and the tank sidewall is limited to something on the order of 2 or 3 inches.
In FIG. 1 of the Bodley patent, the construction is disclosed for covered floating roof tanks. There is a fixed position external roof covering the internal roof to which the pneumatic tube is attached. This type of construction typically has a small seal and is quite limited.
The inventor is also aware of U.S. Pat. Nos. 2,297,985 and 3,255,914, issued to G. Rivers and J. W. Nelson, respectively, which disclose fluid filled seals. The first-mentioned patent discloses a seal comprising a canvas-like material which is attached to the deck of a floating roof. However, the seal is attached in a manner which defines a spillway through which the material used in the seal can flow. Fluid communication with the stored fluid is thereby established, producing several drawbacks.
With regard to the Rivers patent, it is noted that the liquid containing element disclosed by Rivers is attached to the roof at two elevationally spaced apart locations. As disclosed by Rivers, the upper end of the bag is open to the environment, and, as such, the sealing liquid is subject to evaporation with time and/or the collection of leaves, debris and other materials detrimental to the operation of the seal.
In addition, the scuffband material of the Rivers patent is not attached at its bottom end, but is merely draped across the seal. Friction between the scuffband and the tank wall is created by the loading of the scuffband by the liquid's pressure. The friction will tend to slide the scuffband from between the tube and the tank wall.
In addition, the Rivers scuffband does not hold the liquid containing member. The container walls are the structural elements holding the sealing fluid. Hence, the requirements of the seal construction must take into account this loading as well as the liquid containing ability, and therefore a fabric reinforced rubber, for example, is required.
The Nelson patent discloses a toroidal seal for a floating roof comprising a tube which is partially filled with fluid. The tube includes a pair of attaching tabs which are secured to roof mounted structure to attach the seal onto the roof. The Nelson seal includes a one-piece construction having a double wall on part of the circumference which forms a scuff surface. The attaching tabs are located at specific positions on the seal so the scuff surface will be properly oriented when the seal is attached to the roof. Special mountings are positioned on the roof to attach the seal to the roof.
While the seal disclosed by Nelson does overcome many of the drawbacks associated with metallic seals and some of the drawbacks mentioned with reference to the Bodley seal, the Nelson seal, along with all of the other seals known to the inventor, has several very serious drawbacks. These drawbacks are especially evident with the seal disclosed by Nelson, and will be discussed with reference to that patent, but it is to be understood that the drawbacks apply to the other above-discussed patents, as well as to the other seals known in the art prior to this disclosure.
In order to properly orient the Nelson seal on a floating roof, that seal must be moved across and about the roof prior to attachment. During movement of the seal during installation, especially the single ply portion thereof, the seal is very susceptible to being lacerated or otherwise punctured, thereby vitiating the integrity and sealing function of the seal. The seal must therefore be patched or otherwise mended prior to use. Such mending is tedious, difficult and often expensive. Some very large seals may have as many as one hundred punctures, thereby vitiating the integrity of the seal to a degree which may require replacement of the seal even before it is ever used. Because of the aforediscussed structure, tubular seals, such as the Nelson seal, often become twisted during installation, thereby creating additional points of wear, although the mounting of the Nelson seal may overcome any seal twisting problems. Such situations are clearly undesirable.
A further drawback to such tubular seals arises due to the difficulty in splicing such seals. By splicing, it is meant coupling two adjacent ends together. Such splicing occurs when two seals are coupled together, or when one seal is closed during the attachment thereof to a roof. Splicing is also required when a section of a seal is replaced.
Splicing of tubular seals is an extremely onorous procedure, often requiring several man hours to complete. With regard to mending such seals, it is noted that such seals are usually installed in the field by personnel involved in the erection of the tanks. Such personnel are usually not skilled in the art of working with elastomers, adhesives and the like. This fact makes any such mending expensive and questionable. All of the seals disclosed in the above-mentioned patents are subject to this difficulty due to the tubular nature thereof during installation, and due to the configurations thereof which require exact relative orientation of the two ends to be spliced. Any misorientation may result in puckering, wrinkling, or other seal integrity damaging condition. Precise alignment of two tube ends to be joined is especially difficult in the Nelson seal which has a double wall thickness on a portion thereof.
It is also noted that dimensioning of known seals on a tank has difficulties inherent in the design of such seals. Such difficulty is inherent because a length of flat material is being folded to surround a cylindrical tank. This difficulty is further exacerbated if the material is stiff or reinforced rubber, or the like. Such difficulties result in a seal which requires longer material length at the contact of the seal with the tank wall as compared to a length required at the mounting radius. These different circumferences must be accommodated by the pneumatic tube. The only way this accommodation can be effected is by stretching the tube material (restricted by fabric), or puckering, or other such distortion of the pneumatic tube. Such distortion results in gaps or other sealing difficulties. The geometry of the pneumatic seal further complicates this distortion.
Further, with regard to the Nelson patent, the tubular membrane is not only a liquid containing member, but also a load bearing member. Accordingly, in some instances, the weight of the liquid sealing member will be transferred to the mounting through the walls of the seal. The lower support means may not be able to support the weight of the tube and sealing liquid. The entire weight thereof is thus supported by an angle member and that weight is then transferred to that angle member via the seal tube walls. Such construction greatly limits the choice of materials, and virtually eliminates the use of a non-reinforced material.
Further, the double ply portion disclosed by Nelson is likely to require some adhesive bonding of two sheets of materials. If such bonding is used, the bonding adhesive is subject to chemical attack from the sealing fluid and for the stored product. Additionally, the interface between the two plys is subject to the formation of bubbles, blisters and the like.
Because of the structure of known seals, such seals have a limited volume available for containing the fluid. Thus, displacement of fluid is limited, thereby limiting the effectiveness of the seal in accommodating tank wall irregularities. Further, because of the limited volume, accommodation of thermal expansion and/or contraction of the liquid therein is limited, and venting structures are often required. Because of the requirement of longitudinal seams, seals such as those disclosed in the just-discussed patents are usually limited to small dimensions and are thus used only on internal floating covers or roofs. A non-pressurized pneumatic seal of this type simply cannot be made with large dimensions.
The inventor is also aware of the following U.S. Pat. Nos. 2,735,573, 3,135,415, 2,968,420, 3,154,213, 2,973,113, 3,228,551, 3,014,613, 3,228,702, 3,059,806, 4,014,454, 3,116,850.
Accordingly, there is need for a floating roof seal which is easily installed, spliced and which satisfies stringent gap requirements, even for tanks having some wall irregularities.